Recently, an internal combustion engine for an automobile is provided with a variable valve mechanism such as a variable valve timing mechanism, and a direct injection engine varies a fuel injection timing according to a combustion mode in order to achieve high output, low fuel consumption, and low emission. Valve seating noises of intake/exhaust valves and driving noise of a fuel injector are superimposed on an output signal of a knock sensor which detects knocking vibration. Since arising timing of these noises varies according to a control condition of the internal combustion engine, it is difficult to distinguish the knock from these noises.
As shown in JP-2007-9814A (US-2007/0012090A1), vibration waveform components of a plurality of frequency bands where waveforms peculiar to knocking vibration arise are extracted from the output signal of the knock sensor by a plurality of band-pass filters. These vibration waveform components of the plurality of frequency bands are synthesized. Based on the composite vibration waveforms, it is determined whether a knock exists.
As described above, since a timing at which the noise is superimposed on the output signal of the knock sensor is varied according to the control condition of the engine, even if the vibration waveform components of a plurality of the frequency bands where waveforms peculiar to knocking vibration arise are extracted from the output signal of the knock sensor in a knock determination range (a crank angle range from TDC in a power stroke to ATDC 90°CA), the noise may be superimposed on any of the vibration waveform components of the plurality of the frequency bands, which are extracted in the knock determination range.
Since the vibration waveform components of a plurality of frequency bands which are extracted from the output signal of the knock sensor are simply synthesized, if a noise is superimposed on the vibration waveform component of any frequency bands, the noise is superimposed on the composite vibration waveform as it is, which make difficult to distinguish the knock from the noise.
JP-2006-169996A (U.S. Pat. No. 7,181,338B2) shows a countermeasure in which it is determined whether a noise exists every frequency band, and when a noise is detected in any frequency bands, the knock determination is prohibited. However, if the knock determination is prohibited when a knock actually occurs, such a knock cannot be detected.